Diagnosis: Other Pulmonary Function Tests – Lung Disease

Pulmonary function tests (PFT) are a battery of tests that measure lung function and aid in the management of patients with respiratory disease. They are performed using standardized equipment and can be used for diagnosis, prognostication, management and follow-up of patients with pulmonary pathology. Although PFT may not identify the exact pathology, it broadly classifies respiratory disorders as either obstructive or restrictive. In this article, the role of PFT in the measurement of lung mechanics and diagnosis of various diseases will be discussed in detail.

00:00
Right. Lung-function tests: the volumes.
00:03
So the capacity of the lung can also be measured.
How big are the lungs? And this is reallymainly important to identify people who have
shrunken lungs – that happens in pulmonaryfibrosis. So they have reduced total lung
capacity. But it’s also useful in obstructiveairways disease. Because obstructive airways
disease – whether it’s COPD or ongoingsevere asthma – causes an increase in the
residual volume. And as the volume of thelung is dictated by the residual volume as
well as you vital capacity, then the totallung capacity will go up in these patients.
00:39
So you get this paradoxical situation where
patients with airways disease – such asCOPD – will have low vital capacity but
high lung volumes. And that’s largely becausethey’ve got residual volume increases. They
have air trapping in their lungs and thatmakes their lungs bigger than they should
be.
00:59
So what do we mean by air trapping? Well,
essentially as you breathe out, you generatea positive pressure. And that squeezes the
air out of the lungs. But it also squeezesthe tubes taking the air out of the lungs
– the bronchial tree. And if the bronchiare compressed by that positive pressure,
then air flow’s going to be limited. Andin patients who have airways disease, the
problem here is that, on inspiration, theycan breathe in relatively freely but, on expiration,
the obstruction is made worse by this positivepressure. And you get a degree of air trapping
with every breath. And that slowly but surelyinflates the lungs and leaves them operating
with a high residual volume. And this is particularlypronounced in emphysema because the alveolar
destruction that occurs in emphysema allowsthe airways to collapse when faced with this
positive pressure on expiration. That’scalled dynamic airways collapse.
01:57
So we can use flow volume loops to give a
little bit more information about how thepatient’s air flow is changing during the
inspiratory and expiration cycles. They’requite complex. They give distinctive patterns
with different diseases. And so they’reespecially useful in emphysema and in patients
with upper airways obstruction.
02:24
I’ll show you the pictures that we might
get in those circumstances. So, on the left,we have somebody with emphysema. So in expiration,
we do get this dynamic airways collapse. Andhow that is presented in this is that the
flow on expiration is initially rapid andthen, suddenly, there’s a fall. And that’s
due to the dynamic airways collapse. And that’sfollowed by a long tail or slow, low air flow
expiration. If you have an obstruction tothe major airways – so a tracheal tumour
for example – then you get this distinctsquared-off appearance to the flow volume
loop. And that can be very helpful in identifyingthese patients who are otherwise hard to identify.
03:05
So the lung-function tests we’ve described
so far describe the volumes of the lung andhow much air you’re able to shift during
inspiration and expiration and the rate withwhich you shift that.
03:16
What they don’t measure is how easy it is
for oxygen to get from the alveoli into theblood. And to measure that we use the transfer
factor.
03:24
This is measured by inhaling a low concentration
of carbon monoxide and then measuring howmuch carbon monoxide is exhaled. And the difference
between the two is a factor that reflectshow efficient oxygen transfer will be across
the alveolar membrane into the pulmonary capillaries.
03:48
There are two factors we measure: one is the
DLCO – which is the absolute factor. Butthe problem with that is that, if you remove
somebody’s left lung and left them withjust one lung, you’re going to halve the
transfer factor because you’ve halved thesurface area which they have for oxygen diffusion
or carbon monoxide diffusion. So you needto adjust the transfer factor for the alveolar
volume. And that’s called KCO. And that’sthe transfer factor divided by alveolar volume.
And that reflects how efficient oxygen transferwill be per units of lung. And using that
parameter, you can identify patients who actuallyseem to have lung disease as opposed to just
decreased lung volume.
04:28
There are a variety of factors that affect
transfer factor but these fall into the followingcategories. One is the alveolar surface area.
So if you reduce the alveolar surface areathen you’re going to reduce the transfer
factor. And that can happen after a lung resection,as we just discussed, but it also occurs in
emphysema characteristically where the alveolidestruction decreases the surface area available
for gas exchange.
04:53
If you increase the alveolar barrier thickness
– and that occurs in interstitial lung disease– then you will reduce the ability of oxygen
to cross that barrier. So that is also anothercause of a low transfer factor.
05:08
But transfer factors critically depend on
blood supply to the lung. And if you reducethe blood supply to the lung, then it will
go down. And that occurs in pulmonary emboli,pulmonary hypertension and also right-to-left
shunts.
05:21
It’s also critically dependent on the ability
of that oxygen to pick up oxygen and thatrequires haemoglobin. So the transfer factor
is low in patients who are anaemic.
05:31
Cardiac function also is relevant of him.
So mitral valve disease, pulmonary oedemaetc. will also affect the transfer factor.
05:42
So we can actually measure how much oxygen
is present in the blood. And we do two measurementsfor that: one is the oxygen saturation which
measures how saturated the haemoglobin iswith oxygen. Low saturation = low oxygen availability.
High saturation = good oxygen availability.
05:58
In general, people should have saturations
of 95 to 99%. Because of the steepness ofthe oxygen saturation curve, as you become
more hypoxic and the saturation falls to 90%,you get a lot of fluctuation in the values
here. So, although in patients who have 95%+the saturation remains relatively stable,
when you get down below 90% they often fluctuate3 or 4%. It’s a very useful test because
it’s non-invasive and you can use it actuallycontinuously to monitor how patients are doing.
And it’s also useful not just in acute circumstancesbut also for the monitoring of patients who
may be developing respiratory failure dueto chronic lung disease. It’s inaccurate
if the patient has poor peripheral perfusion– if they’ve got cold fingers – and
there’s thick nail varnish on as it measuresthrough the nailbed itself.
06:55
The other measure of oxygen are blood gases.
And what that measures is how much oxygenis dissolved in the blood. But in addition
to that, it gives you other parameters whichare very important: it gives you the carbon
dioxide level dissolved in the blood, it givesyou the pH and it gives you bicarbonate and
the base excess. So it’s very useful inidentifying acid-base problems as well and
problems with excretion of CO2 – hypoventilation.
07:23
So we use blood gases to see whether somebody
has type-I or type-II respiratory failure.
07:28
That is either hypoxia with no increase in
the carbon dioxide or somebody with hypercapnia.
07:33
And if they have type-II respiratory failure,
we need to know whether they have acidosiswith that or not. And it’s also used when
acid-base disorders may be present.

About the Lecture

The lecture Diagnosis: Other Pulmonary Function Tests – Lung Disease by Jeremy Brown, PhD is from the course Introduction to the Respiratory System.

Included Quiz Questions

Which of the following statements is ACCURATE?

Peak flow rate is measured in L / minute.

Peak flow rate is measured in L / second.

FEV1 is measured in L / minute.

FVC is measured in L / second.

Which of the following is the CORRECT measure of total lung capacity?

Forced vital capacity and residual volume.

Forced expiratory volume.

Inspiration reserve volume.

Expiratory reserve volume and tidal volume.

Tidal volume and residual volume.

What is seen in patients with COPD?

High lung volume.

Low inspiration reserve volume.

Normal vital capacity.

High vital capacity.

Low lung volume.

In pulmonary function testing tracheal tumors give which of the following flow volume graph appearance?

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